The present invention relates to an apparatus for the detection of chemical equalization processes in aqueous solutions, in particular for the determination of pH values and/or redox potentials, with an essentially cylindrical probe body, provided with at least one inlet and one outlet opening for calibrating or flushing liquids, which open into a calibrating chamber in the forward area of the probe body, and an immersion pipe axially displaceable in a bore of the probe body and including a protective cylinder at its front end into which an electrode is set, with the head of said electrode being located within a basket like perforated area of the protective cylinder.
Numerous chemical reactions take place in an aqueous solution. In particular in industrial chemistry it is of enormous economical and ecological importance to be able to detect reaction processes, which usually are equilibrium processes, by means of measuring techniques. For this purpose, the pH value and/or the redox potential are conventionally determined in aqueous solutions. Thus, for example, the accurate detection of end points by means of pH and or redox potential determination in the diazotization of amines makes possible titration in the initial operating mixture. This yields information concerning the input quantities of the reagents effectively required. By the resulting nitrite addition optimized relative to minimum requirements it is possible to largely avoid the generation of nitrous gases and thus of NO.sub.x emissions. PH values and redox potentials of solutions in industrial chemistry are usually determined by means of pH or redox probes, inserted through openings provided in the reaction vessel. In a known apparatus a rubber coated steel probe is inserted into the reaction vessel; it is capable of carrying up to three measuring electrodes on its tip. Steel probes of this type are very cumbersome and heavy. For the replacement and the calibration of the electrodes it must be raised by a winch from the vessel, following the release of a screw fastening. Other, lighter steel probes carry one electrode only, but are simpler and may be installed and removed more rapidly. However, for replacement and electrode calibration, they must also be screwed out of the vessel. Furthermore, the light configuration gives rise to stability problems in the case of greater lengths.
The German utility patent DE-U-87 06 309 describes a measuring probe permanently protruding into the reaction vessel. The calibrating chamber may be closed only by means of an additional closure cover relative to the vessel. If the closure mechanism is defective, no calibration of the electrode is possible and the safe replacement of the electrode is not assured. DE-A-3 525 401 describes a probe making it possible to insert and remove the electrode into and from the reaction vessel. However, in order to safely replace the electrode, additional closure elements (valves) must be actuated, which creates an additional safety risk.
The probes described in DE-A-3,118,771 and EP-A-271,764 permanently protrude into the reaction vessel. In order to calibrate or to wash the electrode a sleeve member is slipped onto the electrode, thus creating a calibration chamber. Additional valves are necessary to separate the calibration chamber from the surroundings. The sleeve member cannot be fully drawn into the body of the probe, so that a dead space is present, in which residues may be deposited. This represents an enormous safety risk especially in the case of diazotizing reactions which often involve explosive residues.
A significant improvement relative to the known rod probes is provided, for example, by the interchangeable probes described in the prospectuses No. 776-2-D-300.087 CB and No. 777-1-D of the Ingold Messtechnik GmbH Co., Siemensstrasse 9, D-6347 Steinbach/TS. These interchangeable probes are substantially smaller than the rod probes described above and are mounted laterally over the bottom of the reaction vessel. An immersion pipe with the electrode may be inserted into and removed from the vessel, even during production runs. This provides the preconditions for automatic electrode calibration and automatic reaction control. To replace the electrode, it is simply removed from the immersion pipe and another inserted. If the immersion pipe is withdrawn from the reaction vessel, the opening in the immersion pipe toward the reaction vessel is closed by a protective cylinder provided on the tip of the probe. However, these probes are not always tight in all of the positions of the immersion pipe. Particularly during the passage of the immersion pipe through its intermediate position, liquid from the vessel may briefly be ejected into nonsealing areas of the probe, for example into the calibrating chamber or flushing lines. In particular, in case of a failure of control air, the pneumatically operated immersion pipe may occupy an undefinable position between its terminal inserted and retracted positions and the vessel liquid, which frequently is under pressure, may escape out of control. In diazotizing reactions, special care must be taken to avoid deposits in dead spaces of the probe, as said deposits are often explosive.
It is therefore an object of the present invention to improve an interchangeable electrode of the above described type so that the problems cited are eliminated.